Instruments for measuring the transmission or absorption of radiation through materials contain five key components: a stable source of radiation, a way to restrict the radiation to select wavelengths of interest, a sample interface, a detector to convert radiation to a measurable signal, and a signal indicator. With fluids, a sample cell may be used as the sample interface to contain the fluid during exposure to the radiation. Such a sample cell must be compatible with the fluid, must provide interface surfaces that are transparent to the radiation, and must be in alignment with the radiation. The sample cell may be located directly in the instrument with each sample being automatically or manually inserted, or the sample cell may be located remotely with radiation being conducted to and from the sample cell. Remote sample cells allow for the radiation to be transmitted or absorbed by the fluid at a position at or near the source of the fluid, thus reducing or eliminating sample transportation and time lag problems.
Absorption spectroscopy requires that the sample of fluid contained within the sample cell be free of impurities or contaminants that would interfere with the radiation transmission or absorption measurements. Problems arise since fluid streams to be analyzed frequently contain such contaminants. For example, it is common to find water, particulates, or other emulsified materials in gasoline. These impurities act as scattering agents that reduce the transmission of radiation through the sample and result in high or noisy baselines that degrade the analytical measurement. Therefore, to effectively use current sample cells with fluids containing such contaminants or impurities, a sample conditioning system must be employed. Typical sample conditioning systems include filters, coalescing filters, membrane separators, adsorbents, and the like. Unfortunately, sample conditioning systems require additional investment and maintenance costs and periodically experience failures. Furthermore, sample conditioning systems may severely restrict the flow velocity of the fluid resulting in excessive lag time and rendering real-time analysis and control impossible.
The present invention provides a fluid sample cell that eliminates the need for sample conditioning systems when the base fluid to be measured is distinguishable from the contaminants by a significant difference in density (specific gravity) and is especially useful where the base fluid and the contaminants are in different physical states. The design of the fluid sample cell incorporates centripetal acceleration in order to separate materials of different densities, thus permitting interference-free measurement of the lighter or the heavier component. By forcing the fluid sample to travel in a circular path, the less dense components will be forced toward the center of rotation, due to buoyancy, leaving the more dense components farther from the center of rotation. If the measurement is to be made on the more dense components, the radiation is directed to the outer portion of the sample cell, and if the measurement is to be made on the less dense components, the radiation is directed to the inner portion of the cell.